The UK and Bulgaria are investigating possible connections between the Salisbury nerve agent attack and a 2015 poisoning in Bulgaria.

One of the three Russian suspects linked to the poisoning of ex-Russian spy Sergei Skripal is alleged to have been involved in the poisoning of the Bulgarian owner of an arms factory three years earlier.

Russian citizen Sergei Fedotov made three trips to Bulgaria in 2015 - including once in April, which is when Emilian Gebrev was poisoned, Bulgarian chief prosecutor Sotir Tsatsarov said.

Mr Gebrev survived the attack, but a Finnish laboratory has been unable to identify the poison that was used.

Image:Bulgarian arms dealer Emilian Gebrev was poisoned in 2015

Britain's ambassador to Sofia, Emma Hopkins, said she had discussed the possible poisoning of Mr Gebrev in a meeting with Bulgarian Prime Minister Boyko Borrissov.

She told reporters on Monday: "We are working in a joint team and a close partnership, and we are going to find out the facts in this case.

"All questions about the national security of the UK and Bulgaria are of paramount importance to us, and we will continue this investigation even after Brexit."